Driving a single coil latch relay with a bistable multivibrator

[Beo]

I have the following circuits working each on their own, but I can't get them to work together. Is there a good way to use transistors to operate as a Source/Ground SPDT switch for driving this relay? Any other tricks I can try?

[PRR]

> I can't get them to work together

How does the relay trick work? By dumping big pulses of current into the relay.

Can the flip-flop pass big pulses of current? No. The pull-up is 3.9K. Pull-down is not much better.

Without actually re-thinking what you are doing, I propose:

[Beo]

As soon as I hook up the totem pole to power and ground, the flip flop then only switches between 1.8v and ground. When I hook the flip flop output to the bases, the flip flop drops to 0.8v and no longer flip-flops. I'm using 2n5087 and 2n5088 for the totem pole.

Looks like a lot of current if flowing though the totem pole. my 78l05 got very hot.

[PRR]

Buffer is not wired correctly. Keep track of B, C, E, NPN PNP.

[Beo]

Buffer is not wired correctly. Keep track of B, C, E, NPN PNP.

Sweet! You helped me get it to work! I did check the datasheets for pinouts, but I got my 5087 and 5088 backwards. Also, the multivibrator is gone, as my signal to drive it is from a remote momentary switch using a CD4013 flip flop. By taking the held signal from pins 2/5 instead of the momentary signal from pin 1, I get the push-pull output to hold its state. No current drain because of the cap (which of course sends the return coil pulse when the push pull goes to ground). The smallest cap I could use to drive my relay coil was 10uF.

This is seriously awesome. A 5v reg, two transistors, a cap and a SC Latching relay... that's it! Thanks PRR!

In case anyone is interested, these relays are on sale right now for 99 cents!
http://www.goldmine-elec-products.com/prodinfo.asp?number=G2011

[Beo]

Is it possible to configure a flyback (snubber) diode for a single coil latching relay, or does this make no sense, since current can flow in both directions? Are there other circuit protection or pop eliminator tricks for using a single coil relay for effects switching?

[PRR]

> a flyback (snubber) diode for a single coil latching relay, or does this make no sense

A) Makes no sense, it swings both ways. (I guess you could use two Zeners in anti-series.)

B) Don't worry about it. This relay coil is small, probably won't blow even the small 2N508x parts. (500mA parts cost the same and will surely take the jolts.)

[Beo]

Can you recommend any higher current npn and pnp transistors? Would this be BC series, or MPSA's?

[Beo]

Can you recommend any higher current npn and pnp transistors? Would this be BC series, or MPSA's?

To answer my own question, 3904/3906 is better than 5087/5088, with 200mA max current vs 100mA. Even better is 4401/4402 with up to 600mA, and 2222/2907 with 800mA. Darlingtons might work, MPSA13 can handle 500mA but searching for a PNP complement came up with MPSA64 which I hadn't heard of before. As always, check datasheet for your particular transistor manufacturer.

Follow-on question: with the push-pull transistor configuration, can I still use a small capacitor to slow down the transition? Just between the top collector (V+) and base? Or can I use a cap from base to ground as well to slow down the other direction?

[PRR]

You don't need Darlington current gain or voltage losses.

You don't want to "slow" the action on a pulse relay.

[Beo]

Thanks PRR. You've helped a ton!